An image sensor of a semiconductor device is a device that converts an optical image to electrical signals. The image sensor has a light receiver that receives light from the outside and converts it to electrical signals. A photodiode is a representative type of light receiver. When external light is incident on a depletion region in the photodiode, electron-hole pairs are generated, and the external light is converted to electrical signals.
Spectral sensitivity is one of crucial characteristics of an image sensor. The higher the percentage of incident light absorbed by the photodiode, the higher the spectral sensitivity. An increase in the spectral sensitivity leads to an increase in light sensitivity of the image sensor to external light, giving the image sensor favorable properties. However, there are many factors that can reduce the spectral sensitivity of the image sensor, such as the configuration of the image sensor, the wavelengths of incident light, etc. For example, many specific elements of an image sensor, such as its lines, can prevent light from being freely incident. Also, the amount of light absorbed by the photodiode can be reduced according to the wavelength of the light. For example, with a longer wavelength, light penetration through the photodiode increases. Thus, regions of light with a long wavelength (in the red spectrum, for example) are likely not to be absorbed completely by the photodiode, thus reducing the spectral sensitivity.